1. Field of the Invention
The present invention generally relates to an optical-communication network comprised of a plurality of nodes and, in particular, to a photoelectric cross-connect system included in each node.
2. Description of the Related Art
In general, an optical-communication network comprises a back-up opticaltransmission path in case the active optical-transmission line malfunctions or is down due to errors thereby providing uninterrupted transmission. FIG. 1 is an architecture illustrating a conventional mesh structure of an optical-communication network. As shown, the conventional optical network includes four nodes, 110, 120, 130, and 140, connected using a bi-directional optical-transmission line, first line 115, second line 125, third line 135, fourth line 145, and fifth line 150. In this type of network, one node always has at least one optical-transmission path to the other nodes. The optical-transmission line is an optical fiber connected between two nodes, and the optical-transmission path is a path of the channel transmitted between two nodes, namely a path covering the nodes and the optical fiber. In addition, a photoelectric cross-connect system included in each node 110, 120, 130, or 140 has a data-rate recognizing function and a clock/data recovery function, thus maintaining transparency for a plurality of specific data rates. That is, each of the nodes 110, 120, 130, and 140 is capable of automatically recognizing an unknown data rate of an input channel and recovering a clock and data.
The node (A) 110 is connected to adjacent nodes (D) 140 and (B) 120 through the respective optical-transmission lines 145 and 115, and communicates with the node (D) 140 through a main channel allocated to the node (D) 140 and the node (B) 120. Here, the main channel allocated between the node (A) 110 and the node (D) 140 is the shortest optical-transmission path between the two nodes, and transmitted in the normal operation through the fourth optical transmission line 145 connected between the node A (110) and node D (140). In the same manner, the main channel allocated between the node A (110) and the node B (120) is a channel transmitted through the first optical-transmission line 115 connected between the node A (110) and the node B (120).
In the normal operation, when a channel having the node D (140) as a destination is inputted externally, the node A (110) transmits through the fourth optical-transmission line 145 allocated to the node D (140). The node D (140) receives data through the main channel and transmits the data to a corresponding node through its own channel.
FIG. 2 is an explanatory view showing a state when an error is generated in the fourth optical-transmission line 145 of FIG. 1. As shown in FIG. 2, if the fourth optical-transmission line 145 between the node A (110) and the node D (140) is down and then data destined to the node D (140) is externally inputted, the node A (110) cannot use the main channel allocated to the node D 140. In this case, the node A (110) receives information that the fourth optical-transmission line 145 has an error through a supervisory channel allocated to the node D (140), then notifies the error through a supervisory channel allocated to the node B (120). The node B (120) in turn switches the main channel allocated to the node D 140 to the back-up channel and communicates with the node D (140) through the back-up channel. The back-up channel outputted from the node A (110) reaches the node D (140) in the order of the first optical-transmission line 115, the node B (120) and the fifth optical-transmission line 150.
Note that the time for recovering the service using the back-up channel at a given node satisfies the following equation [error generation recognition time (a few ten μs)]+[switching time (a few ten ms)]+[error generation notification time through supervisory channel (a few ten ms)]+[data recovery time of redundant channel (a few hundred ms)]=[service recovery time (a few hundred ms)].
As such, the recovery time for N nodes is then increased by N multiplied by the recovery time calculated above. Note that the data-rate recognition time of an input channel takes most of the data-recovery time when using the back-up channel. Such data-rate recognition is performed at the input of a channel having different data rates. If the data rate has been recognized previously, the corresponding back-up channel can output the data directly without wasting time in trying to recognize the data rate. However, if a channel with a different data rate from the previously-recognized rate is received, the time to recognize the data rate is required.
As described above, the conventional optical network has a disadvantages in that, when an error is generated in the optical-transmission line, it takes a long time to recover the service through the back-up channel.